Described are copolymers having a high content of acid anhydride functional groups in the polymeric chain, and a stable free radical polymerization process for making such copolymers.
U.S. Pat. No. 5,322,912, incorporated herein by reference in its entirety, describes a free radical polymerization process for the preparation of a thermoplastic resin or resins comprising heating a mixture of a free radical initiator, a stable free radical agent, and at least one polymerizable monomer compound to form a thermoplastic resin or resins with a high monomer to polymer conversion; cooling the mixture; optionally isolating the thermoplastic resin or resins; and optionally washing and drying the thermoplastic resin or resins. Related free radical processes are also disclosed for the preparation of mixtures and block copolymer thermoplastic resins. Resins prepared by the disclosed processes possess a narrow polydispersity and a modality that is controlled by the selection of a free radical initiator and stable free radical agent addition step or steps.
U.S. Pat. No. 5,449,724, incorporated herein by reference in its entirety, describes a free radical polymerization process for the preparation of a thermoplastic resin that includes heating a mixture comprised of a free radical initiator, a stable free radical agent, and ethylene at a temperature of from about 40° C. to about 500° C. and at a pressure of from about 500 to about 5,000 bar to form a thermoplastic resin. The thermoplastic resin has a molecular weight distribution of from about 1.0 to about 2.0.
U.S. Pat. No. 6,087,451, incorporated herein by reference in its entirety, describes a polymer having groups located at the ends of the polymer chain, which groups are derived from stable free radical compounds. The polymer is of the formula SFR—(R)—SFR, wherein SFR represents a covalently bonded stable free radical group and R represents a thermoplastic resin.
U.S. Pat. No. 6,156,858, incorporated herein by reference in its entirety, describes a process for the preparation of polymer comprising heating a mixture comprised of a free radical initiator, a stable free radical agent, a base selected from the group consisting of inorganic bases and organic bases, and at least one polymerizable monomer compound and optionally cooling, followed by optionally isolating the polymer product.
Copolymers having acid anhydride functionality can be used in a number of ways. For example, the acid anhydride functionality can be used as a grafting site for polymer modification. The acid anhydride functionality can also be used as a carboxylic (COOH) acid site if hydrolyzed. In a toner binder resin, the acid anhydride functionality may be used as an aggregation/coalescence site permitting larger size toner particles to be grown from latex particles.
In forming a copolymer from maleic anhydride (MA) or derivatives thereof, a preferred method is to copolymerize the anhydride with electron donor monomers via stable free radical polymerization. Maleic anhydride will polymerize with other monomers in an alternating fashion due to the charge transfer complex that MA forms with electron donor monomers. Stable free radical polymerization permits the anhydride to be introduced as an alternating unit with the electron donor as a pure block when the electron donor is present in molar excess to MA. In conventional polymerization processes, all the MA polymerizes with the electron donor monomer first and subsequently the remainder of the electron donor monomer may be used up via reaction with itself, and thus a significant amount of polymer may be derived that has no anhydride functionality. Stable free radical polymerization is thus preferred in achieving a more uniform product that is substantially free of polymers lacking acid anhydride functionality. In addition, stable free radical polymerization can be used to produce a block copolymer with alternating units that include the anhydride group, which anhydride blocks can be used in preparing further block copolymers with additional polymerizable compounds that may otherwise be incompatible with the donor material.
Currently, however, stable free radical polymerization can be used to make copolymers of only very low maleic anhydride content of, for example, less than 5% by mole of total materials. This is because at the high temperature conditions, e.g., 100° C. to 160° C., of stable free radical polymerization, maleic anhydride and the donor material thermally initiate to cause massive exotherms of greater than 100° C. in a few seconds. That is, bulk or neat free radical polymerization processes of the prior art are prone to generating excessive quantities of heat since the polymerization reaction is exothermic, and as the viscosity of the reaction medium increases, dissipation of heat becomes more difficult. This is referred to as the Trommsdorff effect as discussed and illustrated in Principles of Polymerization, G. Odian, 2nd Ed., Wiley-Interscience, N.Y., 1981, page 272, the disclosure of which is entirely incorporated herein by reference. This exothermic nature of free radical polymerization processes severely restricts the concentration of reactants or the reactor size upon scale up. In other words, it is currently necessary to restrict the amount of anhydride providing material in the polymerization process in order to avoid dangerous and explosive overheating of the reaction, which restriction is typically achieved by dilution with solvents (expensive) or by limiting the amount of MA in a bulk polymerization of electron donor monomer (limiting the anhydride content in the final polymer).
What is still desired is a stable free radical polymerization process that is safe and effective for forming polymers in a bulk process or without solvents that include a greater degree of acid anhydride functionality therein.